I know in the book Ned, Drogo, Viserys, and Robert all end up dead. So I was just curious if they continue to cover the same amount of the book in future episodes, how long can we expect to see those characters?

Viserys will obviously die in the sixth episode. The episode title makes that clear. Ned will certainly die in the season finale. How about Drogo and Robert? Which episodes are they likely to die in? It's been a while since I've read the first book.

Edit to add: refreshing my memory of the book, it looks like Drogo will die in the season finale, too.